Camshaft sensors are used in a motor vehicle to determine the position of the various cylinders in the engine combustion cycle, namely to determine whether each cylinder is in the admission phase, the compression phase, the combustion phase or the exhaust phase. These sensors generally comprise a magnetic field generator (for example: a permanent magnet), a means of detecting the magnetic field (Hall-effect cell, magnetoresistive MR cell, giant magnetoresistive GMR cell, etc. for example) and an electronic circuit for processing the signal received by the means of detecting the magnetic field. These sensors, which are referred to as active sensors, deliver a digital signal to a central processor for processing.
The magnetic field generator may also be the target, made of a magnetic material, exhibiting an alternation of south poles and north poles. In that case, the sensor may or may not incorporate a permanent magnet, depending on the means of detection used. Hereinafter, the south and north poles will be likened to the teeth and troughs of a magnetic target.
In the known way, a camshaft sensor is associated with a target attached to a camshaft. This target takes the form of a disk, the periphery of which is toothed. These teeth have the same height but different spacings (troughs) and lengths, so as to perform encoding (known per se) of the position of the cylinders in the combustion engine combustion cycle for a motor vehicle.
The means of detecting the magnetic field, which is present in the sensor, detects the passage of the teeth of the target past it and the resulting signal makes it possible to determine the position of each signal with respect to the engine combustion cycle, in a way known per se.
In order to determine the position of each cylinder in the engine cycle the curve of the variations in magnetic field perceived by the camshaft sensor during a revolution of the target is observed. This curve exhibits a series of humps, each corresponding to one tooth of the target. By measuring the spacing between each of the humps and the duration of each, it is possible to determine the position of each cylinder with respect to the engine combustion cycle. In order to do this it is therefore important to guarantee the precision of the position of the electrical wavefronts of the signal generated by the sensor with respect to the position of the mechanical wavefronts of the target. Since each of its electrical wavefronts is indicative of the passage of the mechanical wavefronts of the tooth, the objective is to reduce to a minimum the phase shift in the signal caused by the fact that the sensor and the target are variably separated from one another. The electrical signal generated by the sensor changes state (high or low) when the mechanical signal crosses a predetermined switching threshold proportional to its amplitude. In order to do this, this switching threshold is fixed (at 75% of the amplitude, which corresponds to an optimum with regard to the precision between the electrical/mechanical wavefronts for most existing targets) in order to determine the instant at which each wavefront defining a tooth passes. Thus, as soon as a first maximum and a first minimum of the perceived magnetic field are detected, it is determined what switching threshold value corresponds to 75% of this amplitude, and it is considered that a falling front is being detected if the measured value of the magnetic field drops below this threshold value and, conversely, that a rising front is being detected if the measured value of the magnetic field rises above this switching threshold value (or vice versa).
By proceeding in this manner, the moment of detection of the front is optimized. However, this method presupposes that all the teeth have the same height and that there is no defect in geometry (sensors and target). Now, the sensors have the disadvantage of being sensitive to the positioning of the target on the camshaft and to the geometry of this target.
For cost reasons, the targets which are simple pieces of metal equipped with teeth of predetermined dimensions and predetermined spacings, are mass-produced and often exhibit imperfect geometry. In particular, the teeth do not always have the same height in relation to the center of the target. This defect is what is referred to as “out-of-roundness”. It has the effect that the upper part of each tooth of the target is not positioned on the same circle centered on the camshaft. Hence the term “out-of-roundness” used to describe this problem. An out-of-roundness of the mounting of the target on the camshaft may be added to this out-of-roundness in the manufacture of the target. There are also defects with the air gap between the sensor and the target, these defects varying with time and being sensitive to temperature.
Of course, because the camshaft sensor measures variations in the magnetic field created by the passage of the teeth past it, if one tooth is lower (or taller) than the others, the separation between this tooth and the sensor varies in comparison with the other teeth and leads to a variation in the detected magnetic field. These variations in magnetic field may impair the measurements taken (impair the precision of the position of the electric wavefronts in relation to the mechanical wavefronts) or may even fail to be interpreted by the sensor (non-detection of a tooth, the magnetic field being below the switching threshold). The signal delivered by the camshaft sensor is then erroneous and correct determination of the position of each cylinder in the engine cycle is corrupted or even impossible.
In order to alleviate the phenomena of “out-of-roundness” and/or of “airgap defect”, it is known practice in the prior art to calibrate the magnetic field detection means to take account of this “out-of-roundness” and/or of this “airgap defect” and thus deliver a corrected measurement (better electrical/mechanical wavefront precision and elimination of the risk of non-detection of a tooth) to the central processor tasked with determining the position of each cylinder in the engine cycle.
To this end, the switching threshold is recalculated after the passage of the maximum and of the minimum of each new tooth, according to the new amplitude of the magnetic field upon each passage of a tooth past the sensor.
The switching threshold is therefore recalculated after each passage of a tooth, according to the last maximum and the last minimum measured for the magnetic field. However, this method of automatically calibrating a camshaft sensor of the prior art has a major disadvantage: it creates on the sensor output signal disturbances referred to as “jitters” because, effectively, the switching threshold is recalculated and is different for each tooth and, in addition, noise from the sensor and from its amplification sequence is added to the measured magnetic field. As a result, the signal is not repeatable and varies slightly with each revolution of the target. This non-repeatability of the signal is caused, as explained earlier, by the automatic calibration of the sensor in an attempt to alleviate the “out-of-roundness” and/or the “airgap defect”, combined with the electronic noise present on the measurement of the magnetic signal.
The variation (“jitter”) on the output signal from the sensor may, for example, prevent the camshaft timing from being detected, when this is controlled by a VVT (Variable Valve Timing) system.